With the aim of characterizing photoreceptor outer segments and obtaining in situ observation of macromolecular variations due to cell types as well as adaption, we counted the number of outer segment disk membranes using electron micrographs of ultrathin sections as well as intramembrane particles on the complementary replicas of the retina of Lampetra japonica. Long photoreceptor cells (LPCs, cone-type cells) numbered 10000/mm2 in the central as well as peripheral regions, while short ones (SPCs, rod-type cells) numbered 30000/mm2 in the same regions. The LPC outer segment exhibited 306 disks on average during the light cycle versus 364 during the dark cycle. 12.0% of the LPC disks during the light cycle versus 13.4% during the dark cycle represented the "open" disks. The SPC outer segment exhibited 470 disks on average during the light cycle versus 507 during the dark cycle. 11.1% of the SPC disks during the light cycle versus 13.6% during the dark cycle represented the "open" disks. The LPC disk membrane contained 44.3 particles/0.01 μm2 during the light cycle versus 39.5 particles during the dark cycle, 95% of which were derived from the protoplasmic fracture (PF) face. The SPCs contained 36.0 particles/0.01 μm2 during the light cycle versus 43.6 during the dark cycle, 90% of which were derived from the PF-face. The present findings contradict the frequently cited hypothesis that an "open" disk, retaining continuity with the plasmalemma, is preserved characteristically into later stages by the cone outer segment. The significance of the intramembrane particles for the activity of the photoreceptor membrane is discussed.
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